This invention relates to the field of well logging methods and apparatus for determining the characteristics of the earth formations surrounding a bore hole, and more particularly, to a wireline logging cable for safely providing large amounts of electrical power downhole to the well logging tool and for transmitting signals between the surface and the instruments in the well logging tool.
It is often necessary to survey or xe2x80x9clogxe2x80x9d the formations surrounding the borehole by passing a logging sonde or well logging tool through the borehole to measure the parameters or characteristics of the formations at various depths within the borehole. The logging tool is passed through the borehole using a wireline cable which supplies electrical power to the logging tool and transmits telemetry signals between the surface and the logging tool. The logging tool collects data and other information as it passes through the borehole and transmits the data and information to the surface for further processing and analysis.
One type of well logging tool includes a radioactive source housed within the moving tool which emits radiations, such as neutrons or gamma rays, which pass into the formation surrounding the borehole. A portion of the emitted radiation reacts with the formation to produce radiation that is scattered back to the logging tool. The characteristics of this radiation then are transmitted to the surface for identifying the surrounding formation such as oil production zones.
This type of tool typically includes a radioactive source, thereby raising environmental and safety issues. Consequently this tool type is gradually being supplanted by other high-resolution techniques such as Magnetic Resonance Image Logging (MRIL) which do not require the use of radioactive materials. These tools emit high-intensity electromagnetic pulses and measure the relaxation times of atomic nuclei to determine the molecular makeup of the formation surrounding the borehole. MRIL tools generally require more electrical power than the tools employing radioactive-sources.
Another tool which is gradually growing in popularity is called a Reservoir Description Tool (RDT). In uncased boreholes, high-pressure drilling muds are used to prevent the collapse of the borehole walls, and the muds tend to seep into porous formations, making it difficult to measure characteristics of any other fluids present in the formation. The RDT provides a pump to decrease the mud pressure and encourage fluid flow from the formation, and also provides sample chambers to retrieve samples of the formation fluid. The RDT pump motor needs up to 1.8 kW of power to operate properly. The RDT may also be provided with sensing instruments that require a stable power supply and a bi-directional communications channel for telemetry data.
Typically these or other standard well logging tools are passed through oil based well fluids filling the borehole. Many countries are now requiring that for subsea wells the conventional oil-based well fluids be replaced with a salt saturated mud which have a less negative impact on the surrounding environment (e.g. the salt saturated muds will avoid a sheen on the water""s surface). The increased conductivity of these muds shields the formation from many logging instruments, and increased power is required to pass signals from the instrumentation in the well logging tool through these well fluids. Prior methods for supplying downhole power may be inadequate for the increased power demands of xe2x80x9chigh power loggingxe2x80x9d.
One popular standard multiconductor wireline cable comprises six insulated conductors wrapped around a seventh, central insulated conductor. This assembly is encased within two counter-wound layers of steel armor wires which protect the interior conductors and carry the weight of the cable and sonde. A typical series resistance for the insulated conductors is about 10 ohms per thousand feet, or about 300 ohms for a 30,000 foot cable. However, this resistance may be significantly increased due to the higher temperatures which exist in the borehole.
In an effort to minimize the power losses due to resistance in the insulated conductors, conventional high power systems combine four of the conductors in parallel to carry current downhole, and use the armor as a return path. Since the resistance of the armor is about a tenth that of the conductors, or about 30 ohms for a 30,000 foot cable, this results in an overall impedance of about 105 ohms. However, it is expected that with the high power requirements, the armor would often sustain a voltage drop of 130 volts or more, causing a safety hazard. Power delivered in such a manner exposes the workers operating the surface equipment to electrical shock. Additionally, there is a risk of accidents due to electrical arcing downhole, possibly igniting gas.
The present invention overcomes the deficiencies of the prior art.
A system and method is described for safely and economically providing up to 1800 watts to downhole equipment over existing logging cables. If necessary, the power can be further increased by moving to logging cables with lower series electrical resistance. In one embodiment, the system includes a standard multiconductor logging cable which supports orthogonal signal transmission modes on equidistant, circumferentially spaced insulated conductors. The conductors carrying power current are safely enclosed within the logging cable""s armor. A high-power power source on the surface is coupled to the insulated conductors in the cable to drive a power signal on the lowest impedance signal transmission mode (mode M6 for a seven conductor logging cable). Bearing in mind that high-power electrical currents can present a safety hazard, system safety may be enhanced by the addition of a current imbalance detector configured to shut down the high-power power source when currents in the insulated conductors don""t add up to zero. The system may further include multiple power sources operating on different independent signal transmission modes, and may also include multiple telemetry channels which share the power transmission modes via frequency multiplexing.
In another embodiment, the system includes a standard multiconductor logging cable, a downhole toolstring, a programmable power supply, a telemetry receiver, and a computer. The toolstring is powered by a power signal carried on the cable from the programmable power supply. A downhole voltage detector measures the received power signal voltage, and responsively transmits a telemetry signal via the cable to the surface. At the surface, the telemetry receiver converts the telemetry signal into a voltage measurement for the computer. The computer is configured to control the power supply to regulate the received voltage. The system may be provided with multiple power supplies, in which case the computer is configured to analyze the power requirements of the toolstring and to accordingly customize the distribution of power among the independent transmission modes. The computer operates to maximize the power carried by the cable subject to the limitation imposed by the electrical breakdown voltage of the cable. The customization of power distribution may employ, among other things, shifting of signal phases and modification of waveforms.